The present invention relates to a support for patients and particularly to a patient thermal support device that provides an elevated and protected support surface for a patient and that protects and minimizes the disruption of the environment immediately surrounding the patient. More particularly, the present invention relates to a support device that controls the environment immediately surrounding the patient to minimize convective and evaporative heat loss from the patient so that the patient's own body warmth can keep the patient warm. The present invention can additionally be configured to warm a patient if desired using both convective and radiant warming techniques.
Incubators and radiant warmers have both been used to maintain the appropriate body temperature of small or premature infants. An incubator provides a generally transparent enclosure within which heated air is circulated to minimize the heat loss of the patient. In addition, heat is transferred to the patient via convective heat transfer. Incubators are typically provided with a large access door to allow for placement or removal of the infant in the incubator as well as supplemental access ways such as hand ports or small entry doors to permit routine care of the infant while minimizing heat loss from the incubator and the infant.
Radiant warmers provide for continuous and open access to an infant to accommodate a high frequency of intervention by the caregiver. Radiant warmers transfer heat to the patient via radiant heat transfer, typically from infrared heaters which emit infrared energy that is absorbed by the patient. The infrared heater is typically mounted to a support which is suspended above the patient support surface of the radiant warmer. Radiant warmers typically include no canopies or other enclosures that are commonly available on infant support devices to minimize the evaporative water losses of infants because such canopies or enclosures might obstruct the caregiver's access to the infant.
Patients can suffer from conditions that render it desirable to minimize contact between the patient's skin and objects, even including objects such as blankets. In addition, it is occasionally necessary for caregivers to have constant and ready access to the patient in certain critical care situations. What is needed is a patient support device that provides for continuous and open access to a patient while warming the patient should such warming be desired and that can be configured to minimize the evaporative water losses and resultant evaporative heat losses from the patient so that the patient can be uncovered while supported by the device.
According to the present invention, a patient support and environmental control apparatus is provided. The apparatus comprises a frame and an upwardly-facing patient-support surface carried by the frame. In addition, an air curtain generator is mounted to the frame. The air curtain generator provides first and second curtains of air. The patient-support surface has a perimeter and the first and second curtains of air originate adjacent to the perimeter and converge at a point positioned to lie above the patient-support surface. The first and second curtains of air cooperate with the patient-support surface to define a patient space.
A patient can experience heat loss through any of the mechanisms of conductive, convective, and radiant heat transfer, as well as evaporative heat loss that results from the evaporation of moisture from the patient's body. Conductive heat loss accounts for a very low portion of the heat loss of a patient and radiant heat loss can be minimized by heating surfaces such as platforms and walls surrounding the patient. Evaporative and convective heat losses can be controlled by controlling the air near the patient. Factors that operate to influence the extent of evaporative and convective heat losses include the velocity of the air near the patient, the moisture content of the air near the patient, and the temperature of the air near the patient.
The air curtains cooperate with the patient-support surface to define a patient space that is protected from disturbances from outside of the patient space. The air curtains define an effective barrier to atmospheric influences outside of the patient space so that the patient space is generally unaffected by changes in the environment surrounding the patient thermal support device. At the same time, the patient thermal support device can be operated so that there are no physical barriers between the patient and the caregiver, providing the caregiver with continuous and open access to the patient even when the air curtains are in place.
In preferred embodiments, the patient thermal support device in accordance with the present invention uses air curtains to blanket the patient and to create a "thermo-neutral" environment that insulates the patient from heat loss and allows the warmth generated by the patient to keep the patient warm. This device provides caregivers with unobstructed access to patients supported on the platform without the need to cover or in any other manner contact the patient.
A "dry" object can be warmed by blowing dry warmed air onto the object to effect a convective heat transfer. Likewise, a wet object can be warmed by blowing warmed air onto the object. The warming of the wet object can be maximized when the blowing air has a sufficient moisture content that there is no net loss of moisture by the object. However, a patient is more moist than any air that can be delivered to the patient by currently known techniques. As a result, as the velocity of the air engaging the patient increases, the evaporative moisture loss from the patient increases and the evaporative heat loss suffered by the patient increases.
In other words, when warmed air is delivered to the patient there are competing heating effects including a negative heating effect due to evaporative heat losses and a positive heating effect due to the convective heat transfer. For example, when air at 38 degrees C. that is not supplemented by moisture is delivered to the patient at a velocity below approximately 0.15 meters per second (0.49 feet per second), the heating due to convective heat transfer is greater than the heat loss due to evaporative moisture loss so that a net positive heat transfer to the patient occurs. However, when the air delivered to the patient is above approximately 0.15 meters per second (0.49 feet per second), the evaporative heat losses start to work against the convective gains so that at some higher threshold air velocity, the evaporative heat losses withdraw heat from the patient at a faster rate than convection supplies heat to the patient, so that increasing air velocity above the threshold velocity causes a net withdrawal of heat from the patient.
Although the primary purpose of the air curtains is to minimize the disturbance of the cloak of air surrounding the patient, the apparatus provides some convective heating by directing air from at least one additional air curtain toward the patient. The presently preferred embodiment of the patient thermal support device thus includes two opposing air curtains along the sides of the patient-support surface directed upwardly to form an air curtain "tent" above the patient resisting the ingress of air from outside of the patient space through the air curtains and into the patient space. Also, two additional air curtains originating at ends of the patient-support platform directed toward the patient are provided for convective heating of the patient.
In addition, for patients requiring less intervention, the patient thermal support device can be operated in an enclosed mode in which a canopy over the patient-support surface is lowered to engage side walls to enclose the patient space. Moisture can be added to the air curtains to minimize the moisture gradient between the patient and the cloak of air surrounding the patient. Although there is typically a large moisture gradient between the patient and the cloak, this gradient can be minimized by creating a moisture gradient between the air curtains and the cloak so that moisture is transferred from the air curtains to the cloak. Maximizing the moisture content of the cloak minimizes the moisture gradient between the patient and the cloak and minimizes the mass transfer from the patient to the cloak. Thus, evaporative moisture losses and the resultant evaporative heat losses are minimized by minimizing the moisture gradient between the patient and the cloak of air surrounding the patient. This is accomplished in the present invention by adding moisture to the air curtains.
In preferred embodiments, the apparatus also includes several additional features. For example, an exhaust opening at a point spaced-apart from the support surface is provided for withdrawing the air from the air curtains thus enhancing the integrity of the air curtains. The exhaust opening is preferably positioned near an "apex" of the envelope defined by the air curtains when the apparatus is operated in the enclosed mode.
The exhaust opening can be adjacent to the canopy that is positioned to lie above the patient. The canopy and exhaust opening can be vertically adjustable above the support surface so that the distance between the canopy and the support surface can be varied by the caregiver. The apparatus can also be provided with a position sensor for sensing the vertical distance between the exhaust opening and the surface. The air curtain generator can be configured so that the velocity of the air comprising the air curtains automatically varies with the distance between the support surface and the exhaust opening to further enhance the integrity of the air curtains.
The air curtain generator typically includes a channel or manifold containing heated air. The manifold can be positioned adjacent to an underside of a platform holding the patient support surface. The manifold can include an opening or bleeder hole that allows a portion of the heated air to escape and to be directed against a bottom surface of the platform. Heat transferred from the heated air to the bottom surface of the platform also heats the patient support surface through the platform and the mattress, thus providing an additional source of warmth for the patient.
Also in preferred embodiments, the apparatus includes an infrared radiant heater connected to the canopy to transfer heat to the patient via radiant heat transfer. The infrared radiant heater cooperates with the patient's own warmth, the warmed air that escapes the manifold to warm the patient support surface, and the warmed air of the air curtains delivered to the patient, to maintain the desired thermal environment for the patient. In some circumstances, the patient may not generate enough warmth to achieve the desired thermal environment. Also, it may not be desirable to warm the warmed air past a predetermined threshold temperature. The radiant heater can help to achieve and maintain the desired patient temperature when neither the patient nor the warmed air are sufficient for attaining and maintaining the desired patient temperature.
The apparatus in accordance with the present invention is provided with a main controller for controlling the temperature of the patient. The algorithm used by the main controller can control the temperature of the warmed air supplied to the air curtains and the power supplied to the infrared radiant heater. In preferred embodiments, the energy supplied by the radiant heater is minimized to minimize moisture loss due to the infrared energy supplied to the patient.
The algorithm is also designed so that the temperature of the warmed air comprising the air curtains does not exceed a predetermined maximum temperature. When the warmed air temperature approaches this predetermined maximum temperature, the radiant heater starts supplying energy to the patient. If more energy is required, the main controller will increase both the warmed air temperature and the energy provided by the radiant heater until the warmed air temperature reaches the predetermined maximum temperature. At this point, any further temperature increase is provided by the radiant heater. The main controller thus controls the air curtains and radiant heater to manipulate the patient space in order to control the convective and radiant heat transfer to the patient, ultimately to maintain the temperature of the patient at a desired temperature.
According to another aspect of the present invention, an apparatus is provided controlling operation of a patient warming device which includes a support surface for supporting a patient, a convective heater for supplying convective heat to warm the patient, a radiant heater for supplying radiant heat to warm the patient, and a humidifier for adding moisture to air adjacent the support surface. The apparatus includes a controller having a first output coupled to the convective heater and a second output coupled to the radiant heater for varying output power levels of the convective heater and the radiant heater, respectively, to maintain the patient located on the support surface at substantially a preselected temperature. The controller has a third output coupled to the humidifier to adjust an output from the humidifier. The apparatus also includes a temperature sensor having an output coupled to the controller to provide feedback to the controller so that the controller maintains the patient located on the support surface at substantially the preselected temperature. The apparatus further includes a humidity sensor having an output coupled to the controller. The controller adjusts the humidifier based on the output from the humidity sensor to permit the controller to maintain the humidity at substantially a preselected level.
In one illustrated embodiment, the temperature sensor is configured to be coupled to the patient. The apparatus includes an alarm coupled to the controller. The controller generating an alarm signal if the output from the temperature sensor changes above or below a predetermined level from the preselected or desired temperature. The apparatus further includes an input device coupled to the controller to permit a caregiver to adjust the preselected temperature and the preselected humidity level.
In addition to controlling the temperature of the patient, the apparatus in accordance with the present invention can also monitor the level of light to which the patient is exposed and can indicate to the caregiver when the patient is exposed to noise above a desired predetermined maximum noise level. The light monitor system and the noise monitoring system are controlled by the main controller.
Additional objects, features and advantages of the invention will become apparent to those skilled in the art upon consideration of the following detailed description of preferred embodiments exemplifying the best mode of carrying out the invention as presently perceived.